As mobile device technology continues to develop and demand therefor continues to increase, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which have high energy density and operating voltage, long cycle lifespan, and low self-discharge rate, are commercially available and widely used.
As cathode active materials for lithium secondary batteries, lithium-containing cobalt oxides such as LiCoO2 are mainly used. In addition thereto, use of lithium-containing manganese oxides such as LiMnO2 having a layered crystal structure, LiMn2O4 having a spinel crystal structure, and the like and lithium-containing nickel oxides such as LiNiO2 is also under consideration.
Among these cathode active materials, LiCoO2 is widely used due to excellent lifespan characteristics and charge and discharge efficiencies. However, LiCoO2 is low in safety at high temperature and expensive due to resource limitations of cobalt as a raw material and thus there is limitation in price competitiveness.
Lithium manganese oxides, such as LiMnO2, LiMnO4, and the like, are advantageous in that they have high thermal safety and are inexpensive and easy to synthesize. However, such lithium manganese oxides have low capacity, poor high-temperature characteristics, and low conductivity.
Meanwhile, among lithium-containing manganese oxides such as LiMnO2, LiMn2O4, and Li2MnO3 formed through overlithiation of a lithium manganese oxide, Li2MnO3 has very high structural stability while being electrochemically inactive and thus is not suitable for use as a cathode active material for secondary batteries. Thus, the related art discloses a technology of using, as a cathode active material, a solid solution formed using Li2MnO3 and LiMO2 where M=Co, Ni, Ni0.5Mn0.5, or Mn. In such a solid solution used as a cathode active material, Li and O are released from a crystal structure thereof at a high voltage of 4.5 V and thus the solid solution exhibits electrochemical activity. However, there are high possibilities of decomposition of an electrolyte at high voltage and generation of gases and a large amount of a relatively expensive material such as LiMO2 where M=Co, Ni, Ni0.5Mn0.5, or Mn needs to be used and thus such a cathode active material is not in practical use.
On the other hand, LiNiO2-based cathode active materials are relatively inexpensive and exhibit high discharge capacity and thus research into such nickel-based cathode active materials has recently been underway to develop high-capacity batteries. However, crystal structures of these cathode active materials undergo rapid phase transition according to changes in volume caused during charging and discharging cycles and, when exposed to air and moisture, stability of these cathode active materials is rapidly reduced.
Thus, nickel-based lithium transition metal oxides, nickel of which is partially substituted with other transition metals such as manganese, cobalt, and the like, are proposed. These nickel-based lithium transition metal oxides substituted with other metals exhibit relatively excellent cycle characteristics and capacity characteristics at an operating voltage of 4.15 V or less. However, when such nickel-based lithium transition metal oxides operate at a voltage of 4.3 V or higher, problems, such as rapid deterioration of cycle characteristics due to poor structural stability, and the like, have yet to be addressed.
Therefore, there is an urgent need to develop a cathode active material that exhibits high capacity characteristics and addresses structural stability problems.